The observation of strongly interacting many-body phenomena in atomic gases typically requires ultracold samples. Here we show that the strong interaction potentials between Rydberg atoms enable the observation of many-body effects in an atomic vapor, even at room temperature. We excite Rydberg atoms in cesium vapor and observe in real time an out-of-equilibrium excitation dynamics that is consistent with an aggregation mechanism. The experimental observations show qualitative and quantitative agreement with a microscopic theoretical model. Numerical simulations reveal that the strongly correlated growth of the emerging aggregates is reminiscent of soft-matter type systems.

We observe that when an ultracold ground state cesium (Cs) atom becomes bound within the electronic cloud of an extended Cs electronic orbit, ultralong-

We report the observation of cold Cs Rydberg atom molecules bound at internuclear separations of R ∼3-9 µm. The bound states result from avoided crossings between Rydberg atom pair interaction potentials in an applied electric field. The molecular states can be modified by changing the applied electric field. The molecules are observed by mapping the radial separation of the two Rydberg atoms as a function of time delay between excitation and detection using the Coulomb repulsion of the ions after pulsed field ionization. Measurements were performed for 63D + 65D, 64D + 66D, 65D + 67D, and 66D + 68D pairs. The experiment is in good agreement with calculations of the pair interactions for these states. PACS numbers: 34.50.Cx,32.80.Ee,82.20.Bc Frozen Rydberg gases [1,2] have been the subject of intense research recently. The construction of fast quantum gates and single photon sources using dipole blockade [3,4,5,6,7,8,9,10], the study of cold Rydberg atom molecules [11,12,13,14], and the investigation of many body physics [1,2] are central motivations for this work. Cold Rydberg atom molecules are exciting because of the interesting properties that these objects possess. Molecules formed by two cold Rydberg atoms are called macrodimers since the atoms are bound at distances > 1 µm. It has been suggested that due to their delicate nature, macrodimers can be used to study vacuum fluctuations, quenching in ultracold collisions [12] and Rydberg atom interactions including their controllability with applied electric fields. Prior experiments have been unable to unambiguously confirm that these unique states of matter exist as bound states [15,16]. We report the first experimental observation of bound macrodimers which have one of the largest, if not the largest, molecular bond observed to date.The macrodimers that we observe result from avoided crossings between Rydberg atom pair interaction potentials in an applied electric field, ǫ [13]. These FIG. 1: Integrated atomic ion yield spectra and pair potentials for 65D + 67D with ǫ = 190 mV cm −1 . The excitation laser intensity is ∼ 500 W cm −2 . All fine structure and Ω are plotted. Ω = mj1+mj2 is the projection of the angular momentum on R. The dashed line highlights one of the features studied in this paper. macrodimers are unique because they feature a quasicontinuum of bound states, are found at extremely large internuclear separation, R ∼3-9 µm, and are formed by the interplay between Van der Waals interactions and the Stark effect resulting from ǫ. ǫ can be used to stabilize, destroy or modify the potential well that gives rise to the bound states. A spectrum and calculated potentials [17] for the 65D + 67D pair with ǫ = 190 mV cm −1 , are shown in Fig. 1. The potentials have prominent wells at R ∼3-9 µm. These potential wells support hundreds of bound states [13] with maximum energy spacings of ∼100 kHz. The lifetimes of the molecules are limited by the radiative and blackbody decay of the atoms [13]. DETECTING MACRODIMERSMacrodimers are difficult to detect. P...

We observe ultralong-range blueshifted Cs(2) molecular states near ns(1/2) Rydberg states in an optical dipole trap, where 31≤n≤34. The accidental near degeneracy of (n-4)l and ns Rydberg states for l>2 in Cs, due to the small fractional ns quantum defect, leads to nonadiabatic coupling among these states, producing potential wells above the ns thresholds. Two important consequences of admixing high angular momentum states with ns states are the formation of large permanent dipole moments, ~15-100 Debye, and accessibility of these states via two-photon association. The observed states are in excellent agreement with theory.

The theory is developed for one and two atom interactions when the atom has a Rydberg electron attached to a hyperfine split core state. This situation is relevant for some of the rare earth and alkaline earth atoms that have been proposed for experiments on Rydberg-Rydberg interactions. For the rare earth atoms, the core electrons can have a very substantial total angular momentum, J, and a non-zero nuclear spin, I. In the alkaline earth atoms there is a single, s, core electron whose spin can couple to a non-zero nuclear spin for odd isotopes. The resulting hyperfine splitting of the core state can lead to substantial mixing between the Rydberg series attached to different thresholds. Compared to the unperturbed Rydberg series of the alkali atoms, the series perturbations and near degeneracies from the different parity states could lead to qualitatively different behavior for single atom Rydberg properties (polarizability, Zeeman mixing and splitting, etc) as well as Rydberg-Rydberg interactions (C5 and C6 matrices).